This invention relates to an improved method for ionization electrostatic plating (ion plating) and to a device therefor.
It has heretofore been known that the plating of a given article with an electrostatic coating material is accomplished by causing glow discharge between a cathode which is the article subjected to plating and an anode which is the source of coating material, so that the coating material is vaporized and ionized to be deposited on the surface of the article. According to this method, the coating material deposited on the article is scattered because of the phenomenon of cathodic sputtering, causing the plating efficiency to be degraded seriously. Further, the ionization electric current and the sputtering action cooperate to heat the article serving as the cathode and elevate its temperature. Naturally the article subjected to plating is required to be maintained at a fixed temperature level while the coating material is being deposited on the article. When the temperature is too high, however, there ensue undesirable phenomena such as alteration of quality in the deposited coating material and also the substrate. To preclude such phenomena, the article must be maintained at a proper plating temperature during the plating operation. Said temperature elevation, however, interrelates with such factors as electric potential applied, electric current for ionization, pressure of rare gas and kind and shape of the article subjected to plating. The interrelation is such that an attempt to lower the temperature of the article subjected to plating entails a remarkable degradation of the ionization efficiency, for example. It is, therefore, impossible to lower the temperature alone without affecting other factors. It is similarly difficult to provide desired control of the ionization efficiency alone without affecting other factors.
When an article made of such non-metallic material as glass is mounted on the cathode and subjected to plating according to the aforementioned conventional method, the article is converted into a cathode because of the metallic film formed thereon during the initial stage of plating. Consequently, the original cathode and the newly formed cathode are electrically connected by the medium of the film to generate abnormal discharge, which obstructs ideal plating.
The primary object of this invention is to provide a highly efficient method for ionization electrostate plating wherein the temperature of the article subjected to plating can be controlled over a wide range so as to allow great freedom in the choice of coating conditions and permit coating of both metal and non-metal articles.
Another object of this invention is to provide a method for ionization electrostatic plating wherein the cathode is heated to promote emission of thermoelectrons thus promoting ionization and broadening the range of operable coating conditions.
Still another object of this invention is to provide an apparatus for working the above mentioned method.